Secondary batteries typically include a positive electrode, a negative electrode, and a separator sheet interposed therebetween. The separator sheet performs the functions of electronically insulating the positive electrode from the negative electrode, and holding an electrolyte. For example, conventional lithium-ion secondary batteries often include a micro-porous film, made of polyolefin, as the separator sheet. Also, a separator sheet comprising a polyolefin resin and an inorganic powder, or the like has been proposed (see Japanese Laid-Open Patent Publication No. Hei 10-50287). Such a separator sheet is usually produced by drawing a resin sheet that is obtained by a molding method, such as extrusion.
Recently, to improve the quality of secondary batteries, there has been a proposal to adhere a porous electron-insulating layer to an electrode surface (see Japanese Patent No. 3371301). The porous electron-insulating layer is formed on an electrode surface by applying a slurry comprising a particulate filler and a resin binder onto the electrode surface and drying the applied slurry with hot air. Although the porous electron-insulating layer is used as an alternative to a conventional separator sheet in some cases, it is used in combination with a conventional separator sheet in other cases.
The slurry comprising a particulate filler and a resin binder is usually prepared by mixing a particulate filler and a resin binder with a liquid component, and evenly dispersing the particulate filler in the liquid component by means of a dispersing device, such as a bead mill. As schematically shown in FIG. 3, a conventional particulate filler is composed mainly of spherical or substantially spherical primary particles 31, and a plurality of the primary particles 31 gather by weak van der Waals forces to form an agglomerated particle 30.
Conventionally, in terms of stabilizing the thickness and void ratio (porosity) of the porous electron-insulating layer, efforts have been made to break down the agglomeration of primary particles as much as possible by means of a dispersing device, such as a bead mill, in order to evenly disperse independent primary particles in a liquid component (see Japanese Laid-Open Patent Publication No. Hei 10-106530 (FIG. 2)).